1. Field of the Invention
The present invention relates to a diagnostic apparatus for nuclear medicine in which an accumulated or deposited state of an administered radioactive medicine (radioisotope: RI) in an organ or the like of a living body is detected from the outside of the living body by detecting portions, each constituted by a scintillation detector and a collimator, so as to obtain an image of the state.
2. Description of the Related Art
As a diagnostic apparatus for nuclear medicine of this type, a SPECT (single photon emission CT) apparatus for imaging an RI distribution tomographic image by using a gamma-ray nuclide is known. SPECT apparatuses are roughly classified into an Anger system gamma camera type and a ring type in which single probes are arranged in the form of a ring. The former type can obtain a planar image and a SPECT image (tomographic image), whereas the latter type can obtain only a SPECT image. That is, Anger type SPECT apparatuses are very versatile. The Anger type SPECT apparatuses are classified into a one-detector type, a two-detector type, a three-detector type, and a four-detector type depending on the number of detectors.
A one-detector type SPECT apparatus is of the most versatile type and is mainly designed for planar images. However, an apparatus of this type has low sensitivity.
A two-detector type SPECT apparatus has two detectors arranged to oppose each other and is designed to simultaneously obtain planar images of front and rear surfaces of a target patient or a whole body scan image.
A three-detector type SPECT apparatus can obtain a high-resolution, high-sensitivity SPECT image in a small field of view (head portion) by using a fan beam collimator. In addition, a SPECT image in a large field of view (body portion) can also be obtained by changing the radius (by means of movement toward each other) and using a parallel collimator.
A four-detector type SPECT apparatus cannot use a fan beam collimator but can only use a parallel collimator. For this reason, this type has lower sensitivity than the three-detector type (with the same resolution) in a small field of view (head portion). In addition, the field of view of this apparatus is fixed. Hence, the apparatus is exclusively used for a head portion or a body portion.
In the types of SPECT apparatuses, the three-detector type is superior to other types in versatility because it can obtain high-quality SPECT images in large and small fields of view. However, the three-detector type SPECT apparatus cannot obtain planar images opposing each other at 180.degree., and especially a planar whole body scan image, thus posing a problem.